Coal is still today one of the most widely used fuels for the generation of electricity with several hundred power plants in the United States alone and an even greater number worldwide, utilizing coal combustion to generate electricity. One of the principal by-products from the combustion of solid fuels, such as coal, is fly ash, which generally is blown out of a coal combustor within the exhaust air stream coming from the combustor. Fly ash has been found to be very useful in building materials applications, particularly as a cement additive for making concrete, due to the nature of ash as a pozzolanic material useful for adding strength, consistency and crack resistance to the finished concrete products.
Most fly ash produced by coal combustion, however, generally contains a significant percentage of fine, unburned carbon particles, sometimes called “char”, that reduces the ash's usefulness as a byproduct. Before the fly ash produced by the combustion of coal and/or other solid fuels can be used in most building products applications, it must be processed or treated to reduce residual carbon levels therein. Typically, it is necessary for the ash to be cleaned to as low as 1–2 percent by weight carbon content before it can be used as a cement additive and in other building products applications. If the carbon levels of the fly ash are too high, the ash cannot be used in many of the aforementioned applications. For example, although fly ash production in the United States for 1998 was in excess of 55 million tons, less than 20 million tons of fly ash were used in building product materials and other applications. Consequently, carbon content of the ash is a key factor retarding its wider use in current markets and the expansion of its use to other markets.
In order to lower the residual carbon content of fly ash to appropriate levels, it generally is necessary to ignite and combust the carbon. The fly ash particles, therefore, must be supplied with sufficient temperature, oxygen and residence time in a heated chamber to ignite and burn the carbon within the fly ash particles. Currently, a number of technologies have been explored to try to effect carbon combustion in fly ash to reduce the carbon levels as low as possible. The primary problems that have faced most commercial methods in recent years generally have been the operational complexity of such systems and maintenance issues that have increased the processing costs per ton of processed fly ash, in some cases, to a point where it is not economically feasible to use such methods.
Such current systems and methods for carbon reduction in fly ash include, for example, a system in which the ash is conveyed in basket conveyors and/or on mesh belts through a carbon burn out system that includes a series of combustion chambers. As the ash is conveyed through the combustion chambers it is heated to burn off the carbon therein. Other known ash feed or conveying systems for transport of the ash through combustion chambers have included screw mechanisms, rotary drums and other mechanical transport devices. At the high temperatures typically required for ash processing, however, such mechanisms often have proved difficult to maintain and operate reliably. In addition, such mechanisms typically limit the exposure of the carbon particles to free oxygen by constraining or retaining the ash within baskets or on mesh belts such that combustion is occasioned by, in effect, diffusion through the ash, thereby retarding the effective throughput through the system. Accordingly, carbon residence times within the furnace also must be on the order of upwards of 30 minutes to effect a good burn out of carbon. These factors generally result in a less effective and costlier process.
Another approach to generating carbon combustion in fly ash has utilized bubbling fluid bed technology to affect carbon burn out. In this system, the ash is placed in a bubbling fluid bed supplied with high temperature and oxygen so that the carbon is burned or combusted as it bubbles through the bed. This bubbling fluid bed technology generally requires residence times of the carbon particles within a furnace chamber for up to about 20 minutes or more. The rate of contact of the carbon particles with oxidizing gasses in the bubbling fluid bed also is generally limited to regions in which the bubbles of gas contact solids, such that the rate of contact is related to the effective gas voidage in the bubbling bed, which is typically around 55–60 percent (i.e. around 40–45 percent of solids by volume). These systems have, however, been found to have limited through-put of ash due to effective carbon combustion rates with required carbon particle residence times generally being close to those of other conventional systems.